1. Technical Field
The invention relates to audio arrays, and more particularly, audio bass arrays exhibiting high directivity in sound field generation.
2. Description of the Problem
The capacity to direct and focus the output from an array of loudspeakers is of great interest. Increased control over direction and focus allows an increased proportion of sound energy generated to be delivered to a precise location and reduces the amount of energy arriving at locations of no interest. In effect the efficiency of the overall system is increased.
As used in this patent the term “directivity pattern” of an array of transducers is a graphical description of a generated sound field at a particular frequency in all directions of a specified plane. The “beam width” of a directivity pattern is the angular distance between two points on either side of the principal axis of the sound field where the sound field is down 6 DB from its value, at the particular distance from the source, along the principal axis.
The present patent concerns primarily bass or low frequency sources, used for generation of sound at or below 200-300 Hz. Bass loudspeakers are typically based on a piston driven, diaphragm type transducer set in one face of a enclosure (which may be ported). Unlike higher frequency devices constructed in the same fashion, they operate effectively as simple (i.e. point) sources.
A long history of analysis exists for linear arrays of simple sources. Transducers laid out in a linear array, all operating at a matched frequency and with equal spacing between adjacent units, generate sound waves which either cancel one another, or reinforce one another, in various directions away from the array. To generate a sound field with a principal axis perpendicular to the linear array and minimal sound away from the ends, all of the simple sources are driven in unison. To produce a so-called “end fire array” the phase relationship of the signal to the transducers is varied. An end fire array is one which produces a sound field with a principal axis aligned with the array.
Examples of linear arrays abound in the art, from acoustic doublets to more complex systems. Consideration of acoustic doublets is instructive of the basic principles involved. In a general sense an acoustic doublet is approximated by a simple diaphragm transducer which is freely mounted (i.e. not in a baffle or enclosure). The transducers are spaced by less than one quarter of wavelength for almost any frequency to be reproduced and the faces of the diaphragm are inherently driven at 180 degrees out of phase with one another to produce an “end fire device”. In an acoustic doublet, with the transducers vibrating in opposed phase, the pressure waves cancel one another along axes perpendicular to the axis of alignment of the sources and the principal axis of the sound field generated is defined by the center points of the transducers. While an acoustic doublet is an ideal, the same basic principle applies to more complex systems, optimized for particular frequencies. The arrays of interest here though typically use greater spacing.
U.S. Pat. No. 6,766,033 is an example of a bass array system. The '033 patent provides a mechanism to vary spacing between adjacent pairs of a plurality of transducers among a plurality of predetermined “fixed distances”. A common signal source feeds the transducers along separate channels. Each channel includes a variable time delay. Selection of the time delay, taking into account the spacing between transducers, determines the direction of the principal axis of the sound field and illustrates some of the suppression of side lobes possible. The loudspeaker at the “base” of the array is excited first with loudspeakers “forward” from the base unit timed to generate sound based on the propagation delay from the base unit.
U.S. Pat. No. 6,128,395 is directed to a loudspeaker system with controlled directional sensitivity. A loud speaker array is taught in which identical loudspeakers are arranged in a straight line or planar array and is apparently directed to such an array where hung in what is effectively a free field. The spacing between adjacent speakers is not equidistant, but logarithmically progressive. Filtering of the audio signal and selected delay may be applied to control beam width and direction, respectively. The '395 patent states that “By not making the mutual spacing of the loudspeakers equidistant but adapting it to the frequency requirements, it is possible to control the directional sensitivity (the “transmission angle”) up to, certainly, 8 kHz. The side lobe level is reduced at the same time.” Indeed the '395 patent seems directed to extending directional sensitivity from the 1400 Hz range up to as close to 8 kHz feasible, and thus, for audio purposes, should be considered a high frequency system. The '395 patent allows directional control over the principal axis of the sound field, which allows for the possibility of end fire. The '395 patent discusses controlling what it terms the “opening angle”. The opening angle is defined as the angle through which a sound source can be turned such that the sound pressure does not fall by more than 6 DB with respect to the maximum value which is measured at a fixed point in a plane in which the sound source is located . . . ” in effect, beam width. The opening angle can be selected by choice of the FIR or IIR filter coefficients and the transmission angle can be selected by adjusting delay times.